nbc news' pete williams joins us with all the details. good evening, pete. >> well, the combination of evidence here the government says they have, larry, the main thing here is from a surveillance video from the scene of a second bombing, they say they can see dzhokhar tsarnaev come into view. they see him work his way into the crowd, take his backpack off, lower it to the ground. they say still photographs show it down on the ground at his feet and they say one of the most telling moments is when the bomb at the first site goes off. everyone around him looks, virtually everyone, they say, but he is calm, staring straight ahead, not reacting at all. he briefly glances that way, appears to use his telephone, walks away, 13 seconds later the bomb goes off at his location. that's the photographic evidence. next is the forensic evidence. they say both the bombs were made in pressure cookers. they say the inside they had green hobby fuse and also bee-bees that were somehow had an ad he'sive on them. they owe owe an adhesive on them. they say